Knitted fabrics comprising metal fibers are described in WO97/04152, WO94/01372, WO94/01373 and U.S. Pat. No. 6,756,330.
The use of a textile fabric as separation cloth between mould and glass, to form side-lites and back-lites for automotive business is known. During this contact, temperatures of 400 to 700° C. are used. It is of great importance that no marks are left on the glass surface after the contact of the glass and the textile fabric.
The use of textile fabrics out of 100% glass fibers is known. The disadvantage of these glass fiber cloths is that they do not resist the mechanical action during the glass shaping process. Also the use of textile fabrics, partially or fully consisting out of metal fibers is known. Using these fabrics as mould coverings, the mechanical action of the bending process is withstood better, but there is still the risk of marking the glass, by transferring the woven or knitted pattern into the glass surface which has contacted the textile fabric.
Further, it is known that the use of knitted structures is more suitable to cover moulds, since knitted surfaces can be draped better on moulds and less or no folds will be created when bending the knitted fabric, especially on three-dimensionally shaped surfaces or moulds.
The risk of having marks, caused by use of textile fabrics as the separation cloth for moulds in glass bending processes, is influenced by several parameters, such as glass temperature and pressure used to bend the glass. Since for example the automotive industry requires more complex glass surfaces, that is glasses which show a deeper bend, the glass has to be heated to a higher temperature and the pressure to bend the glass, is increased as well. These two adjustments to the production parameters of the bending process, makes the glass more sensitive to markings since higher temperatures and/or pressures make the glass softer, and creates a more obvious transfer of the textile structure, either woven or knitted, on the glass surface because of the higher pressure.
Another parameter that influences the risk of marking, is the wear of the textile fabric, used as a separation cloth between moulds and glass, due to the repetitive contacts with glass sheets, and the temperature. This temperature makes the fibers become more sensitive to breaking forces, and the mechanical action of the glass sheets against the fabric makes the fabric wear out little by little. Since the fibers which are standing out on the yarn surface will suffer most on this mechanical action, they will disappear after several contacts with glass. The stitches out of which the knitted fabric is made or the weaving pattern used to provide the woven fabric, will be transferred more obviously to the glass surface.
Separation cloths should preferably meet next requirements:
1. The cloth should resist the bending temperature. Typically, these temperatures rise up to 700° C. when the bending takes place in the heated part of the furnace. When the glass bending takes place outside the furnace, the mould temperature will be less, at temperatures of 400 to 500° C.
2. The cloth should be able to follow the mould shape as close as possible.
3. The separation material should show sufficient air permeability. It is taken as a limit that separation cloths should at least have an air permeability of 400 l/dm2/min, and preferably more than 750 l/dm2/min.
4. The weight of the separation cloth is preferably between 600 g/m2 and 2000 g/m2. Fabrics with less weight usually wear out too fast, where too heavy fabrics tend to elongate too much under their own weight, so causing obstruction in the furnace for the glass to pass in the neighborhood of the fabric before or after the bending action.
5. The thickness of the separation cloth is preferably more than 0.8 mm and even more preferred more than 1 mm. Too thin fabrics show a lack of elasticity in the direction perpendicular to the fabric surface.
6. And as already mentioned, the risk for remaining marks on the glass surface should be reduced to a minimum.
The higher the number of requirements met, the better the performance of the separation cloth between mould and glass in the glass bending process will be.